Typical wireless networks, such as networks conforming to one of the IEEE 802.11 standards, are arranged such that a plurality of communications devices each has a wireless connection to a central node. The central node is conventionally described as an access point. Typically, all of the communications devices communicate on the same channel (i.e. in the same range of frequency). This sharing of the communications medium may lead to collisions, which is where two or more communications devices attempt to use a channel simultaneously. Consequently, mechanisms are required to avoid collisions.
One known mechanism for dealing with collisions is known as a “Distributed Coordination Function” (DCF). A DCF works by requiring a communications device wishing to transmit to listen on the channel for a particular interval. If the channel is busy (i.e. another communications device is transmitting on the channel) the communications device defers attempting to access the channel for a “back-off period”. The back-off period typically has a random element, so that communications devices do not synchronously defer access and then re-attempt access.
However, when there are a large number of communications devices, and in particular when there are a large number of communications devices which transmit at irregular intervals, the known techniques for avoiding collisions are inefficient. Moreover, a communications device can waste a significant amount of energy (i.e. battery life) attempting to communicate on a channel, without actually being able to do so.
Therefore, there is a need for improved mechanisms for arranging access to a communications medium.